1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with an apparatus and method for removing a top nozzle as a first step in reconstituting a fuel assembly wherein a plurality of cutters are lowered and raised at the same time to ensure that a plurality of cuts are made in a single common plane. 2. Description of the Prior Art
Conventional designs of fuel assemblies include a plurality of fuel rods and control rod guide thimbles held in an organized array by grids spaced along the fuel assembly length and attached to the control rod guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the control rod guide thimbles which extend slightly above and below the ends of the fuel rods. At the top end of the fuel assembly, the guide thimbles are attached in passageways provided in the adapter plate of the top nozzle. The guide thimbles may each include an upper sleeve for attachment to the top nozzle and the uppermost grid.
During operation of such assembly in a nuclear reactor, a few of the fuel rods may occasionally develop cracks along their lengths resulting primarily from internal stresses, thus establishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. Such products may also be released into a flooded reactor cavity during refueling operations or into the coolant circulated through pools where the spent fuel assemblies are stored. Since the fuel rods are part of an intergral assembly of guide thimbles welded to the top and bottom nozzles, it is difficult to detect and remove the failed rods.
Until recently, to gain access to these rods it was necessary to remove the affected assembly from the nuclear reactor core and then break the welds which secure the nozzles to the control rod guide thimbles. In so doing, the destructive action often renders the fuel assembly unfit for further use in a reactor because of the damage done to both the guide thimbles and the nozzles which prohibits rewelding.
Recently, as described in the first patent application cross-referenced above, a method for reconstituting a fuel assembly has been devised which does not involve breaking the welds which attach the guide thimbles to the adapter plate of the top nozzle. Instead, the guide thimbles are severed from the adapter plate by circumferentially cutting each thimble just below the attachment area. Then, the original top nozzle is removed from the cut guide thimbles. The upper ends of the fuel rods are now exposed from the top of the fuel assembly. Thus, access to the fuel rods is gained for any of a variety of purposes: inspecting them for failure, removing and replacing failed rods, transferring partially spent fuel rods from one assembly to another, and/or rearrangement of fuel rods to attain better uranium utilization in the reactor core. Once, inspection, removal, replacement and/or rearrangement of the fuel rods is completed, a new top nozzle having an adapter plate containing modified guide thimble passageways is placed on the cut guide thimbles and the upper ends of the thimbles are circumferentially bulge fitted into annular grooves formed in the passageways.
While the fuel assembly reconstitution method briefly described above has demonstrated considerable promise as a measure by which domestic and foreign utilities can minimize both operating and maintenance expenses, a need exists for improvements in the manner in which the method is carried out so as to enhance its commercial acceptance.